The work is a continuation of a project that last year demonstrated improved efficiency in lithium-air batteries through the use of noble-metal-based catalysts. The new work creates carbon-fiber-based electrodes that are substantially more porous than other carbon electrodes, and can therefore more efficiently store the solid oxidized lithium that fills the pores as the battery discharges.

Hollow carbon fibers with diameters on the order of 30 nm were grown on a ceramic porous substrate, which was used as the oxygen electrode in lithium-oxygen (Li–O2) batteries. These all-carbon-fiber (binder-free) electrodes were found to yield high gravimetric energies (up to 2500 W h kg discharged−1) in Li–O2 cells, translating to an energy enhancement about 4 times greater than the state-of-the-art lithium intercalation compounds such as LiCoO2 about 600 W h kg electrode−1). The high gravimetric energy achieved in this study can be attributed to low carbon packing in the grown carbon-fiber electrodes and highly efficient utilization of the available carbon mass and void volume for Li2O2 formation. The nanofiber structure allowed for the clear visualization of Li2O2 formation and morphological evolution during discharge and its disappearance upon charge, where Li2O2 particles grown on the sidewalls of the aligned carbon fibers were found to be toroids, having particle sizes increasing (up to about 1 μm) with increasing depth-of-discharge. The visualization of Li2O2 morphologies upon discharge and disappearance upon charge represents a critical step toward understanding key processes that limit the rate capability and low round-trip efficiencies of Li–O2 batteries, which are not currently understood within the field.

Gravimetric Ragone plot comparing energy and power characteristics of CNF electrodes based on the pristine and discharged electrode weight with that of LiCoO2. Source: Mitchell et al.

"We grow vertically aligned arrays of carbon nanofibers using a chemical vapor deposition process. These carpet-like arrays provide a highly conductive, low-density scaffold for energy storage," explains Robert Mitchell, a graduate student in MIT's Department of Materials Science and Engineering (DMSE) and co-author of a paper describing the new findings in the journal Energy and Environmental Science.

During discharge, lithium-peroxide particles grow on the carbon fibers, adds co-author Betar Gallant, a graduate student in MIT's Department of Mechanical Engineering. In designing an ideal electrode material, she says, it's important to "minimize the amount of carbon, which adds unwanted weight to the battery, and maximize the space available for lithium peroxide," the active compound that forms during the discharging of lithium-air batteries.

"We were able to create a novel carpet-like material — composed of more than 90 percent void space — that can be filled by the reactive material during battery operation," says Yang Shao-Horn, the Gail E. Kendall Professor of Mechanical Engineering and Materials Science and Engineering and senior author of the paper. The other senior author of the paper is Carl Thompson, the Stavros Salapatas Professor of Materials Science and Engineering and interim head of DMSE.

In earlier lithium-air battery research that Shao-Horn and her students reported last year, they demonstrated that carbon particles could be used to make efficient electrodes for lithium-air batteries. In that work, the carbon structures were more complex but only had about 70 percent void space.